1. Field of the Invention
The invention relates to the field of optical fibers for wavelength division multiplex (WDM) transmission networks. The WDM transmission networks referred to are more particularly networks known as “metropolitan area networks”. The transmission distances in metropolitan area networks are typically from a few kilometers, for example 10 km, to approximately 300 km.
2. Description of the Prior Art
The invention aims to provide an optical fiber for use in the above type of network embodying a very good compromise that principally encompasses, for the transmission distance concerned, non-linear effect constraints leading to distortion in the signal conveyed, signal loss constraints leading to attenuation of the signal conveyed, and constraints associated with the cost of production of the optical fiber concerned and the transmission system as a whole. To be more precise, the invention aims to provide a relatively simple optical fiber that is inexpensive to produce, has a high effective area, typically greater than 50 μm2 at the wavelength of 1,550 nm, thereby diminishing the impact of non-linear effects, and is usable over the greatest possible portion of the combination of bands S. C and L, respectively extending from 1,460 nm to 1,520 nm, from 1,520 nm to 1,565 nm, and from 1,565 nm to 1,625 nm. The invention also aims to provide an optical fiber that is monomode in bands S, C and L, and which has good resistance to non-linear effects in those bands of the spectrum. The invention further aims to provide an optical fiber that can be used even in a transmission system with no means of compensating the chromatic dispersion of said optical fiber. Over most of the combination of bands S, C and L, and preferably over the whole of all three bands S, C and L, the chromatic dispersion must remain sufficiently low to necessitate no compensation of chromatic dispersion and sufficiently high to induce no non-linear effects significantly degrading transmission. In a preferred embodiment, the invention aims to provide a simple optical fiber that is inexpensive to produce, has an effective area greater than 50 μm2 at the wavelength of 1,550 nm, and a dispersion from 2 to 9 ps/(nm*km) over a range of wavelengths from 1,460 nm to 1,625nm, combined with acceptable bending losses. Acceptable bending losses are advantageously below 100 dB/m for a diameter of 20 mm at a wavelength of 1,625 nm and below 30 dB/m for a diameter of 20 mm at a wavelength of 1,550 nm.
Thus the invention aims to provide a monomode optical fiber that can be used over a wide range of the spectrum, minimizes non-linear effects, does not necessarily necessitate chromatic dispersion compensation when used in a metropolitan area network, and also has acceptable bending losses. To this end, the optical fiber according to the invention has an optimized narrow range of chromatic dispersion, a maximum dispersion slope threshold, a maximum chromatic dispersion to dispersion slope ratio threshold, and one or two claddings that are sufficiently deeply buried.
The optical fibers comprise a core of varying index profile and a cladding of constant index. The core comprises a plurality of slices that can have different shapes, in particular a rectangular, triangular, trapezoidal or alpha shape, and can further be truncated and/or include a pedestal. The invention aims to provide a simple optical fiber that is inexpensive to produce. To this end, the optical fiber according to the invention is restricted to three or four slices. A profile with three slices is the simplest in structural terms, but makes it obligatory to bury the only buried slice deeply, whereas, with the structurally less simple profile with four slices, each of the two buried slices can be either buried less deeply, with an equivalent effective area, or buried very deeply, with an increased effective area. The central slice is raised, in contrast to coaxial profiles which have a large central recess. Again with the aim of providing a simple optical fiber that is inexpensive to produce, according to the invention the dispersion slope chosen must not be too low, as otherwise the optical fiber becomes too costly to produce. The minimum dispersion slope threshold chosen is an important aspect of the invention, providing the opportunity to achieve the required compromise, which would otherwise be too difficult to achieve. A preferred minimum threshold slightly higher than the previous one facilitates the production of an optical fiber in accordance with the invention having optical properties comparable with those obtained close to the lowest minimum threshold.
The narrow chromatic dispersion range claimed, the narrow dispersion slope range claimed, the sufficiently deeply buried character of the buried slice or slices relative to the height of the central slice, the maximum zero dispersion wavelength threshold, and the limited number of slices, constitute essential features of the invention for achieving the good compromise that is required for optical fibers primarily intended to be used in a metropolitan area transmission system. Other optional preferred features further improve on said good compromise, in the direction of a very good compromise or even an excellent compromise.
None of the prior art cited and analyzed hereinafter has all the features of the invention or achieves an optimum compromise like that at which the invention is aimed.
Example 1 of French patent application FR 0002316 discloses an optical fiber that has in particular the disadvantage of a dispersion slope that is too low to achieve the required compromise, the unit cost of an optical fiber being inversely proportional to the dispersion slope for low values of the dispersion slope, and a very low dispersion slope leading to extreme sensitivity of the optical properties to even slight variations in the index profile of the optical fiber.
Example 4 in international patent application WO 02/14919 discloses an optical fiber which has in particular the disadvantages of a dispersion slope that is too high and buried layers that are insufficiently deeply buried to achieve the required compromise.
Example 1 of European patent application EP 1189082 discloses an optical fiber that has in particular the disadvantages of a dispersion slope that is too high and buried slices that are insufficiently deeply buried to achieve the required compromise.
Example 5 of European patent application EP 1189082 discloses an optical fiber that has in particular the drawbacks of a chromatic dispersion that is too low and buried slices that are insufficiently deeply buried to achieve the required compromise.
Example 10 and trial sample 3 of European patent application EP 1189082 disclose an optical fiber that has in particular the drawbacks of a dispersion slope that is too high, a chromatic dispersion that is too low, and buried slices that are insufficiently deeply buried to achieve the required compromise.
Comparative examples 4, 9, 10, 11 and 12 of European patent application EP 1211533 disclose an optical fiber that has in particular the drawbacks of a chromatic dispersion that is too high and buried slices that are insufficiently deeply buried to achieve the required compromise.
Example 18 of European patent application EP 1146358 discloses an optical fiber that has in particular the drawbacks of a chromatic dispersion that is too high and buried slices that are insufficiently deeply buried to achieve the required compromise.
Example 22 of European patent application EP 1146358 discloses an optical fiber that has in particular the drawbacks of buried slices that are insufficiently deeply buried to achieve the required compromise and bending losses significantly greater than those of the optical fiber according to the invention.
Examples 1 and 3 of European patent application EP 1130828 disclose an optical fiber that has in particular the drawback of an effective area at 1550 nm that is too low; moreover, no profile is specified.
Examples 2 and 4 of European patent application EP 1130828 disclose an optical fiber that has in particular the drawback of a dispersion slope that is too low; moreover, no profile is specified.
Examples 2 and 3 of European patent application EP 98117828.8 disclose an optical fiber that has in particular the drawback of buried slices that are insufficiently deeply buried to obtain the required compromise and appears to have bending losses significantly higher than those of the optical fiber according to the invention.
Example 4 of European patent application EP 98117828.8 disclose an optical fiber that has in particular the drawbacks of a chromatic dispersion that is too low and buried slices that are insufficiently deeply buried to achieve the required compromise.
Example 5 of European patent application EP 98117828.8 discloses an optical fiber that has in particular the drawbacks of a dispersion slope that is too high, a chromatic dispersion that is too low and buried slices that are insufficiently deeply buried to achieve the required compromise.